Honors Chemistry

Thursday, February 4, 2010

Daily Quiz - Drawing dot structures for covalent molecules.

Covalent Bond Properties

Bond Order refers to the number of shared pairs of electrons between two atoms. If two atoms share one pair of electrons, a single bond, the bond order is 1. If two atoms share two pairs of electrons, a double bond, the bond order is 2.
For a given pair of atoms, a higher bond order results in a shorter bond length and a higher bond energy. For example, a C to O single bond is 143 pm in length with an average bond energy of 358 kJ/mol. A C to O double bond is 123 pm in length with an average bond energy of 745 kJ/mol. A C to O triple bond is 113 pm in length with an average bond energy of 1070 kJ/mol.
Electronegativity (EN) is the relative ability of a bonded atom to attract the shared electrons. There is a trend in electronegativity of elements. Electronegativity is highest at the top of a group of elements and to the right of any period of elements.

Because atoms of different elements pull on shared electrons with different forces, sometimes a covalent bond involves an unequal sharing of electrons.

When the two atoms making the covalent bond pull with equal strength (they have the same value of electronegativity), represented by diagram A, the shared pair of electrons remains in the middle between the two atoms. Neither atom dominates. This type of covalent bonding is called a nonpolar covalent bond.
When the two atoms making the covalent bond pull with different strengths (they have different values of electronegativity), represented by diagram B, the shared pair of electrons spend their time closer to one side of the bond and away from the other end. One atom dominates. This type of covalent bonding is called a polar covalent bond.
A polar bond results in one side of the bond having a partial negative charge (the side that pulls more on the electron pairs) and the other side with a partial positive charge. This formation of partial charges creates a dipole among the atoms forming the bond. One atom is more electronegative and the other is more electropositive.
If the difference in electronegativity values of the two atoms is greater than 1.7 then one of the atoms pulls much more than the other, and an ionic bond results (as represented by diagram C).

Daily Quiz

B Period

Review definition of bond energy.
Use average bond energies to calculate the enthalpy change of a chemical reaction.

G Period

Covalent Bond Properties

Bond Order refers to the number of shared pairs of electrons between two atoms. If two atoms share one pair of electrons, a single bond, the bond order is 1. If two atoms share two pairs of electrons, a double bond, the bond order is 2.
For a given pair of atoms, a higher bond order results in a shorter bond length and a higher bond energy. For example, a C to O single bond is 143 pm in length with an average bond energy of 358 kJ/mol. A C to O double bond is 123 pm in length with an average bond energy of 745 kJ/mol. A C to O triple bond is 113 pm in length with an average bond energy of 1070 kJ/mol.
Electronegativity (EN) is the relative ability of a bonded atom to attract the shared electrons. There is a trend in electronegativity of elements. Electronegativity is highest at the top of a group of elements and to the right of any period of elements.

[[image:file/view/ElectronegativityTrends.gif width="570" height="271" align="center"]]

Because atoms of different elements pull on shared electrons with different forces, sometimes a covalent bond involves an unequal sharing of electrons.

[[image:file/view/138187_Hsc-1401-03.jpg.jpg width="537" height="104" align="center"]]

When the two atoms making the covalent bond pull with equal strength (they have the same value of electronegativity), represented by diagram A, the shared pair of electrons remains in the middle between the two atoms. Neither atom dominates. This type of covalent bonding is called a nonpolar covalent bond.
When the two atoms making the covalent bond pull with different strengths (they have different values of electronegativity), represented by diagram B, the shared pair of electrons spend their time closer to one side of the bond and away from the other end. One atom dominates. This type of covalent bonding is called a polar covalent bond.
A polar bond results in one side of the bond having a partial negative charge (the side that pulls more on the electron pairs) and the other side with a partial positive charge. This formation of partial charges creates a dipole among the atoms forming the bond. One atom is more electronegative and the other is more electropositive.
If the difference in electronegativity values of the two atoms is greater than 1.7 then one of the atoms pulls much more than the other, and an ionic bond results (as represented by diagram C).

B Period

Worksheet of bond energies and calculating enthalpy changes.
Use dot structures and electronegativity arrows to determine if a molecule is polar or nonpolar.

Ammonia

Hydrogen chloride (HCl)

No homework for Monday.